1. Field of the Invention
The present invention relates in general to a method of fabricating reflective type liquid crystal display (LCD). In particular, the present invention relates to a method of fabricating a concave/convex profile on a reflective film of the reflective type LCD.
2. Description of the Related Art
Currently, liquid crystal display (LCD), having the advantages of low power consumption, thin type, light weight and low driving-voltage, has been put to practical use, such as with personal computers, archival treatments, navigation systems, Gameboys, projectors, view finders and portable machines (watches, electronic calculators and televisions). In particular, a reflective type LCD can display bright images and high contrast by the reflection of the incident light, and thereby achieves lower power consumption, thinner type and lighter weight.
Mostly, the reflective type LCD employs the twisted nematic (TN) crystals or the super-twisted nematic (STN) crystals. However, the polariscope disables more than half of the illuminant and therefore darkens the brightness of the display images. In order to enhance the use efficiency of the illuminant, now the reflective type LCD employs the guest-host (GH) type crystals, that is, a combination of the nematic crystals and dichroic dyes, to change the arrangement of the crystal molecules and adjust the light-absorbency of the dyes by a electric field. Also, for further enhancing the brightness and contrast, a reflective film with a gently curved profile serving as a white scattered surface is needed because the concave/convex on the upper surface of the reflective film can increase the intensity of the incident light.
As shown in FIG. 1, an LCD 10 according to the prior art comprises a semiconductor substrate 1 and a metal wiring layer 2 formed on the substrate 1. The metal wiring layer 2 electrically connects elements (not shown) formed in the substrate 1 for driving the LCD 10, such as transistors, resistors or capacitors. A polyimide resin layer 3 is coated on the semiconductor substrate 1 and has a contact hole 4 for exposing a predetermined area of the metal wiring layer 2. A display electrode 5, made of aluminum and serving as a reflective film, is formed on the polyimide resin layer 3 and connects to the metal wiring layer 2 through the contact hole 4. The polyimide resin layer 3 has an upper surface with small protrusions or depressions having a gently curved profile, and accordingly the display electrode 5 has an upper surface with small protrusions or depressions having a gently curved profile for making a white scattered surface. The LCD 10 further comprises a transparent glass substrate 8 and a transparent conductive film 7 formed on the transparent substrate 8. A Guest-Host type liquid crystal 6 is filled in a gap between the display electrode 5 and the transparent conductive film 7.
However, if the protrusions or depressions of the display electrode 5 have sharp-pointed edges, the incident light generates multiple reflective effects and thus darkens the brightness of the display images. In order to solve this problem, the U.S. Pat. No. 4,519,678 provides two methods of fabricating the gently curved profile of the display electrode 5. In the first method, as shown in FIG. 2, the polyimide resin layer 3 is firstly coated on the semiconductor substrate 1, and then cured by a heat treatment about 100xcx9c200xc2x0 C. Next, by using a mask 9 with a spot-like resist pattern wherein the openings are arranged at a pitch of 1 to 50 microns, an etching process is performed on the polyimide resin layer 3 to form a plurality of small protrusions and depressions having the concave/convex profile. Concerning the materials of the polyimide resin layer 3, if a thermosetting resin is used, it is desirable to perform an isotropic etching process, such as a wet etching method. If a thermoplastic resin is used, it is desirable to perform an anisotropic etching process, such as a dry etching or a reactive ion etching (RIE) method and then a heat treatment about 150xcx9c500xc2x0 C. is necessary for rounding the edges of the protrusions or depressions after removing the mask 9. Next, a photolithography process is needed to pattern the contact hole 4 on the polyimide resin layer 3 for exposing the predetermined area of the metal wiring layer 2. Next, after covering the display electrode 5 on the upper surface of the polyimide resin layer 3, a selective etching process is performed to shape the upper surface of the display electrode 5 into the desired curved profile.
In the second method, as shown in FIG. 3A, an insulating layer 12 is first formed on the substrate 1. Then, as shown in FIG. 3B, the insulating layer 12 is etched to become a plurality of separated bumps 16 by using a mask 14. Next, as shown in FIG. 3C, after removing the mask 14, the polyimide resin layer 3 is coated on the substrate 1 by a high-speed spin coating process at 2000xcx9c5000 rpm. Since the viscosity of the polyimide resin layer 3 differs from the change in speed, the upper surface of the polyimide resin layer 3 shows a concave/convex profile depending on the arrangement of the bumps 16. Finally, as shown in FIG. 3D, the display electrode 5 is deposited on the polyimide resin layer 3 and accordingly has the gently curved profile.
From the above description, the two conventional methods are thought to be too complex to ensure the process reliability. For this reason, a third conventional method provides a simpler process for fabricating the concave/convex profile. As shown in FIG. 4A, a polyimide resin layer 22 with photosensitive and thermosetting characteristics is formed on a semiconductor substrate 20. Next, an exposure process is performed on the polyimide resin layer 22 by using a mask (not shown) with a spot-like pattern, and then a development process is performed to remove the exposed areas of the polyimide resin layer 22 so as to form the upper surface as a plurality of continuous bumps 221 or concaves 222, as shown in FIG. 4B. Next, a heat treatment is necessary for changing the viscosity of the polyimide resin layer 22 and reflowing thereof. At the same time, owing to the polyimide resin layer 22 made of thermosetting resin, the edges of the bumps 221 and the concaves 222 are rounded to become a gently curved profile 22xe2x80x2, as shown in FIG. 4C. Finally, another heat treatment is utilized to cure the polyimide resin layer 22.
However, by the request of the high aperture ratio, the polyimide resin layer 22 with a thickness of 2xcx9c4 is needed but which always causes each bump 221 to have different thicknesses during the development process. The lack of uniformity will make the profile 22xe2x80x2 have unsymmetrical concaves/convexes during the sequential heat treatment and reflowing. Also, the quality of the reflective film is accordingly reduced.
An object of the present invention is to provide a method of forming a reflective type LCD to solve the above-described problems.
Another object of the present invention is to provide a method of fabricating a reflective type LCD, including the steps of: (a) providing a substrate; (b) forming a polymer resin layer on the substrate; (c) forming a positive-type photoresist layer on the polymer resin layer, wherein the upper surface of the photoresist layer has a convex/concave profile; and (d) performing a dry etching process to completely remove the photoresist layer and partially remove the polymer resin layer so as to shape the upper surface of the polymer resin layer into a convex/concave profile.
Another object of the present invention is to provide a method of fabricating a reflective type LCD, including the steps of: (a) providing a substrate on which a metallic wiring layer is covered; (b) forming a polymer resin layer on the substrate to cover the metallic wiring layer; (c) forming a contact hole on the polymer resin layer to expose a part of the metallic wiring layer; (d) forming a positive-type photoresist layer on the polymer resin layer to fill the contact hole; (e) forming the upper surface of the photoresist layer as a convex/concave profile; (f) performing a dry etching process to completely remove the photoresist layer and partially remove the polymer resin layer so as to make the upper surface of the polymer resin layer have a convex/concave profile; and (g) covering a metallic reflective layer on the polymer resin layer so as to have an upper surface with a convex/concave profile.
It is an advantage of the present invention that the polymer resin layer with the concave/convex profile in the present invention is shaped by etching the photoresist layer without etching and rounding the polymer resin layer. Consequently, the method is simplified and the uniformity of the upper surface of the polymer resin layer is well controlled. This can ensure the property of the formation the reflective film.
This and other objectives of the present invention will no doubt become obvious to those of ordinary skill in the art after having read the following detailed description of the preferred embodiment, which is illustrated in the various figures and drawings.